Xerographic plate and method



Nov. 29, 1966 c. SNELLING ET A1. 3,288,602

XEROGRAPHIC PLATE AND METHOD Filed April 4, 1962 2 Sheets-Sheet l Y l f''I' 3 F l L- T 91 ff @HO-*W1 F/G. l l

i; NWT/l "L orma., d ArToR/VEV Nov. 29, 1966 C. SNELUNG ET AL 3,288,602

`XERC)GRAPHIC PLATE AND METHOD 2 Sheets-Sheet 2 Filed April 4, 1962INVENTORS G V1 WH E LC EV M LATP O LI-To T ND H T SNO. A R mmRmOi El.TGL/ TMDOHH. www HR C Y am B United States Patent O 3,288,602XERGGRAPHIC PLATE AND METHOD Christopher Snelling, Penfield, Robert W.Gundlach, Victor, and George R. Mott and William D. Hope, Rochester,N.Y., assignors to Xerox Corporation, Rochester,

NX., a corporation of New York Filed Apr. 4, 1962, Ser. No. 185,051 18Claims. (Cl. 96-1) This invention relates in general to Xerography andin particular to an improved xerographic plate and improved methods ofxerographic plate sensitization, image formation, development andtransfer.

In the art of Xerography as originally disclosed in U.S. Patent2,297,691 to Carlson and later related patents an electrostatic latentimage is formed on a photoconductive insulating layer with a conductivebacking and is developed through the deposition thereon of finelydivided electroscopic material which is later transferred and fixed to asheet of copy material. According to these inventions thephotoconductive insulating layer with a conductive backing in the formof a fiat plate or cylindrical drum is first charged, to sensitize it,and is then exposed to the image or pattern to be reproduced. Thisexposure renders the illuminated areas of the photoconductor conductiveallowing the charge in those areas to be dissipated while the charge inthe nonilluminated areas is retained. Since there is very little or nolateral conduction in the nonilluminated photoconducting material,charge is retained in image configuration.

Since the time of the original Carlson patent noted above the originalplate charging and adhesive transfer techniques disclosed in that patenthave been largely superseded by more uniform and reproducibleelectrostatic techniques. For example, xerographic plate charging is nowgenerally accomplished by the method disclosed in U.S. Patent 2,588,699to Carlson which involves the use of an ion producing filament orfilament arrays operating on corona discharge principles. This type ofcorona discharge electrostatic technique is also utilized at the presenttime for the transfer of the electroscopic particles representing adeveloped image to a sheet of copying material as disclosed in U.S.Patent 2,576,047 to Schaffert. U.S. Patent 2,945,434 to Eichlerdiscloses the use of both of these techniques in a xer-ographic ofiicecopying machine. Although these electrostatic charging and transfertechniques have been used in xerographic copiers to produce copies ofhigh quality they require the use of relatively -large and delicateequipment employing relatively high voltages.

Although other charging and transfer techniques have been proposed fromtime to time it has been found for one reason or another that they arenot commercially feasible.

In addition to problems arising in applying charge to the xerographicplate and transferring the developed image to the copying material,selection of the materials for the plates has presented somewhat of aproblem because the photoconductive layer must not only be a goodphotoconductor but also a relatively good dielectric so that it willretain the applied charge until it is exposed. This has restricted tosome extent the number of materials which may be used as photoconductorson xerographic plates.

Another problem of Xerography is developing relatively large uniformlycharged areas. The lines of force of these large fields extend outwardof the plate surface at their edges or peripheries whereas they tend torun into the plate towards the conductive plate backing at theircenters. Since the field lines attract the developing material this hasresulted in large concentrations of the charged electroscopic developingparticles at the edges of 3,258,602 Patented Nov. 29, 1966 ICC theselarge charged areas and relatively small amounts of developer in thecenters of these areas making the centers look washed out in the finalcopy.

Accordingly, it is an object of this invention to define novelxerographic plates.

It is a furtherk object of this invention to define novel xerographicplates in which the dielectric and photoconductive functions of theplate are separated.

It is another object of this invention to define novel methods andapparatus of sensitizing xerographic plates.

A further object of this invention is to define novel and improvedmethods and apparatus of electrostatic image formation.

It is also an object of this invention to define novel xerographicapparatus and methods for the transfer of the developed xerographicimages to a copy surface.

It is yet another object of this invention to define novel xerographicplates and techniques of Xerography which result in good developercoverage of large charged areas on the exposed xerographic plate.

It is also an object of this invention to describe a novel xerographiccopying method and device which may be easily and quickly switched frompositive to positive copying to positive to negative copying.

The above and still further objects, features, and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed disclosure of specific embodiments of the invention,especially when taken in conjunction with the accompanying drawingswherein:

FIGURE 1 is a cross section View of a plate according to this invention.

FIGURE 2 is atop view taken along section lines 2-2 of FIGURE l andincluding some external connections not shown in FIGURE l.

FIGURE 3 is a cross sectional View of a xerographic plate comprising asecond embodiment of this invention.

FIGURE 4 is a top view taken along section lines 3-3 of FIGURE 3 andshowing one possible -grid configuration for that embodiment.

FIGURE 5 is a top view showing an embodiment of this invention for usein a cylindrical rotatable xerographic drum.

FIGURE 6 is an end view of xerographic apparatus including a drum of thetype shown in FIGURE 5.

FIG. 6A is a sectional view of FIG. 6 along line 6A.

FIGURE 7 is a side sectional view of a further -improved modification ofthe plate embodiments of this invention.

FIGURE 8 is a showing of simplified appartus illustrating imageformation in accordance with an embodiment of this invention.

FIGURE 9 is a showing of simplified apparat-us illustrating developertransfer in accordance with this invention.

Referring now to FIGURE l of the drawings there is illustrated a plate 1comprising `a supporting base 2 bearing a number of slender conductors4, 5, '6, and 7 which run in parallel lines across the Width of theplate. ASince as will be explained hereinafter it is desired to maintainelectrical separation between the conductors 4, 5, 6, and 7 thesupporting base 2 should be a good insulator land of adequate strengthto act as a support for the plate 1. Since base 2 is mainly for supportpurposes its use is 0ptional and it may be eliminated if support isunnecessary or if other plate elements make up a self supportingelement. The conductors on the supporting base are very slender and thinand are uniformly spaced so that there are from to 350 of them per inchof plate although less or more conductors per inch may also be used.These conductors make up approximately 50% of the area of the plate. Theconductors may be placed on the supporting base by means of photoresistand etch or engraving techniques as explained in connection with theconstruction examples which follow. The conductors are covered with alayer of dielectric 3 adequate to prevent dielectric breakdown when theplate is employed in accordance with the method described below.Materials with a fairly high dielectric constant are preferably usedlhere so as to allow a thin dielectric layer however a Mylar layer with1a relatively high dielectric constant of 3.4 was successfully utilized.

Above the dielectric 3 there is a photoconductive layer 8 which may forexample be any of the known photoconductive insulating layers employedin xerography such as photoconductive pigments in an insulating binder,vitreous selenium, yanthracene, and the like. In this instance, however,the photoconductor need not be an exceptionally good insulator in itsnon-activated condition since in the plate of this invention thedielectric layer 3 rather than the photoconductive layer is used to trapcharge representing the latent electrostatic image remaining afterexposure to the original to be reproduced. This allows the use of binderplates employing binders of less insulating qualities, if desired, asWell as other photoconductors with relatively poor charge retainingproperties. Alternate conductors in the plate such as 4 and 6 areinterconnected beyond the plate periphery while conductors such as 5 and7 separating the alternate conductors are also interconnected beyond theplate periphery, both sets of conductors being kept electricallyseparated. Alternatively, the conductor sets may be mutuallyperpendicular if they are placed at different levels in the dielectricor in the insulating base. If such a base is utilized, the onlyrequirement is that the conductor sets be electrically separated.

Thus, it may be seen that each set of adjacent conductors together withthe dielectric between them makes up a small capacitor in the plate. Thecapacitors may be charged by the application of a potential. Preferably,the dielectric layer 3 should be no thicker than the center to centerspacing of the conductors so as to maximize the capacitor inducedfringing elds reaching the photoconductive layer 8.

FIGURE 1 also shows :a switch 9 for connecting the conductor sets eitheracross a potential source or 58 or connecting them together throughground.

Exposure of the plate may be through the back or support or to the outersurface. In the event that it is desired to expose the photoconductorthrough the supporting base it would be necessary to make the base andthe dielectric layers of a transparent insulating material such asglass, Mylar, or the like. Since the conductors only cover about half ofthe area ofthe plate as explained above these conductors could be opaqueand still allow :approximately half of the exposure entering through theback of.

the plate to reach the photoconductive layer resulting in a halftoneline pattern of the image on the photoconductor. Alternatively, theconductors could be fabricated from a transparent material such as athin layer of tin oxide so that all of the light from the exposure wouldreach the photoconductive layer.

FIGURE 2 is a top-sectional View taken through section lines 2-2 ofFIGURE 1 and `showing an embodiment of conductors 4, 5, 6, 7, etc.,embedded in the plate 1 and connected to an outside potential source 10.

The plate section shown in FIGURE 3 is basically another embodiment ofthe concept disclosed in connection with the description of FIGURE 1above. This embodiment comprises a dielectric layer 12 disposed on aconductive backing 11 which is connected to ground. A conducting screenor grid structure made up of conductors 14, 15, 16, 17, etc., is placedabove the dielectric layer 12 and the upper surface is then overcoatedwith a photoconductive layer 13 similar to that described in connectionwith the FIGURE vl embodiment. In this embodiment, the built-incapacitor is made up of the conductive grid 14-17 etc., electricallyseparated from the conductive backing 11 by dielectric material 12.Since all of the parallel conductors 14-17 are maintained at the samepolarity in this embodiment there is no problem with inadvertentconnections between adjacent parallel conductors as might arise with theFIGURE 1 embodiment where adjacent conductors are held at oppositepolfarities. The grid structure 14-17 etc., may be connected throughswitch 18 either to potential source 19, or 55, or to ground. As in theFIGURE l embodiment the photoconductor and dielectric layer are ratherthin so as to maximize the effects of the fringing iields from thebuilt-in capacitor. In fact, certain alternative grid placements may beused so as to maximize this eld. For example, the grid may be placed ushwith the t-op surface of the photoconductive layer away from theremainder of the plate. With this. modication, devolpment of the platewill take place close to the electric field source where the ield isrelatively strong, thus providing high density images and improvedsensitivity.

FIG. 4 shows conductors 14, 15, 16, and 17 interconnected with otherconductors to make up the grid structure of the FIGURE 2 embodiment ofthe plate. By fabricating the grid in this closed screenpattern theeffects of a break in any one conductor will be minimized becausepotential will be applied through perpendicular conductors and carriedto each side of the break.

Various other screen or grid configurations and placements will occur tothose skilled in the art such as placingV the screen or grid in thedielectric so that it-s top layer is Hush with the surface of thedielectric thereby giving a uniform thickness of photoconductivematerial or making the screen in the form of a spiral with or withoutinter secting Iradial conductors and the like.

Although any one of the novel plates described .above could be chargedor sensitized with conventional high voltage corona discharge techniquesthe following novel methods of plate sensitization and image formationare contemplated for use with these plates. Essentially these novelmethods involve operating the plate as a capacitor while using thephotoconductive layer of the plate as a light responsive switch and thedielectric layer of the plate as a charge trap.

When battery potential is applied across alternate conductors of theFIGURE 1 embodiment by placing switch 9 in the position shown in thefigure a field is set up between the alternate conductors through thedielectric material and the insulating base 2. If the plate is notilluminated and consequently the photoconductive layer 8 is in itsrelatively insulating state the iield will also be set up in this layer.Thus, lines representing this eld would appear similar to lines 20 asshown in FIGURE 1.

With potential applied the plate is then exposed to an image to bereproduced. This renders the illuminated areas of the photoconductivelayer 8 conductive. As is well known an electric eld when applied to aconductor causes the free electrons within it to move in such a way asto make the interior of the conductor a tield free, equipotentialvolume. In this case when the area of the photoconductive layer lyingabove conduct-ors 4 and 5 is illuminated, or subjected to otheractivating electromagnetic radiation, the electric field resulting froman application of potential across the conductors 4 and 5 for example,causes the free electrons within this conducting area of layer 8 to movetowards the positive conductor 5 leaving the area above the negativeconductor 4 relatively positive. In this case the charge -moves throughthe illuminated photoconductive layer until it reaches the interface ofdielectric layer 3. Since the material 3 remains in an insulatingcondition regardless of illumination the charge is stopped at thisinterface. Thus, at the interface of dielectric layer 3 andphotoconductive layer 8 above conductor 4, positive charge is stoppedwhile at the interface above-conductor 5 negative charge is stopped, andwhen the illumination is shut off, these char-ges are trapped at theinterface because photoc-onductor 8 reverts to its insulating statethereby limiting charge mobility. Owing to this rearrangement of chargewithin the photoconductor 8 while in its conducting state and the latertrapping of the charge in its new position when the illumination sourceis shut off the electric field previously created by conductors 4 and 5no longer exists since the photoconductor is equipotential. Assumingthat the area of the photoconductor 8 -overlying conductors 6 and 7 isnot illuminated and consequently is still in its relatively insulatingstate an electric field emanating from conductors 6 and 7 still existsin the photoconductor because the in- -sulating character `of thephotoconductor in this area limits charge mobility thereby preventingthis area from becoming equipotential.

Triboelectrically charged electroscopic developing particles are thenused to develop the fringing fields emanating from the exposed plate.These particles are attracted by the electric field which still existthrough the photoconductive layer in the unexposed areas. It isinteresting to note that either positively charged developing particles,negatively charged developing particles or a combination of both may beused to develop this plate in this condition. Either polarity ofelectroscopic developing parti-cles hereinafter referred to as toner, ora combination of both polarities may be used to develop the platebecause negative toner is attracted t-o the area above positiveconductor 7 while positive toner is attracted to the area aboveconductor 6. Thus, if only positive toner is used only every otherunexposed conductor area is developed. This also holds true for the useof negative toner alone. However, if a combination of positive andnegative toner is used all unilluminated areas above the conductors aredeveloped. Owing to the close spacing of the c-onductors within thedielectric layer it is only necessary to use the two polarities of tonerfor devel-oping Iin the event that continuous tone qual-ity is desired.Since most electrophotographic developers in use today contain a't leastsome toner of each polarity while only toner of one polarity isattracted to the conventional latent electrostatic image duringdevelopment the invention of a novel plate and method utilizingtoner ofboth polarities is of signicant value. If desired, developer containingsubstantial amounts of both polarities of toner may be deliberatelyproduced for use with this invention. For example, when a two elementdeveloper consisting of carrier beads and toner particles is used thetoner may consist of a mixture of two different types of particlesselected so that one type is above and one below the carrier beads inthe triboelectric series. This would result in the carrier impartingpositive charge to one type of .toner and negative to the other. Thisand other methods of producing two polarity toner, any of which may beused in connection with this invention are more fully described in U.S.Patent 3,013,890 to Bixby.

Operation of the plate in the manner explained above results in apositive image from the projection of a positive original. This platemay also be operated so that a negative -image results from a projectionof a positive original. This result is obtained by moving switch 9 tothe grounded position after charging and exposure as outlined abovewhile maintaining the plate in total darkness. This shorts out thealternate conductors through ground while the whole photoconductivesurface 8 is relatively insulating. Since potential no longer existsacross alternate conductors these conductors no longer produce anelectric field in any part of the plate. Thus, the electric fieldcollapses in those areas of the photoconductor which have been exposedto light. However in those areas of the photoconductor which have beenilluminated and consequently were conducting, charges moved so that alarge number of negative charges reside at the interface of thephotoconductor and the dielectric above the positive conductor and alarge number of positive charges reside at the interface of thephotoconductor and the dielectric above the negative conduct-or. Thesecharges are trapped at the interface when the plate illumination is shutoff because the photoconductor reverts to its insulating condition.These trapped charges then set up electric fields of their own of apolarity opposite to that of the original fields through the relativelyinsulating photoconductive layer. These lields may now be developedresulting `in a negative image in a photographic sense of the originallight pattern.

The FIGURE 3 embodiment of this invention may be operated in the mannerdescribed above in connection with FIGURE l; however, a second mode ofoperation which `may be used with either plate embodiment is describedbelow in connection with the FIGURE 3 embodiment.

The plate is first flooded or uniformly exposed to light so as to putits photoconductive layer 13 is in its relatively conductive state.During this lighted period the conductive grid made up of members 14,15, 16, '17, etc., is connected through switch 18 to a source ofpotential such as battery 19, the other end of which is connectedthrough ground to the conductive plate back 11. If, for example, thegrid structure is connected to the positive pole of the potential sourcethe grid is made positive and the conductive backing of the plate ismade negative. Thus, the applied potential acts to charge the capacitormade up of grid 14-17 dielectric 12 and plate 11. For illustrativepurposes an electric fringe field ernanating from this charged capacitoris shown only above conductors 16 and f17. When potential is firstapplied to the grid structure a field such as that shown aboveconductors 16 and 17 exists through the illuminated photoconductivelayer 13 and the dielectric layer 12. Fringe fields such as the oneshown do not exist for very long across the whole surface of theilluminated plat while it remains in its conducting state since thefield causes rearrangement of the charge or free electrons in thephotoconductor resulting in rapid field collapse as the photoconductorapproaches its equipotential condition. In this case the field causeselectrons to move towards the top of the positive conductors and rendersthe interface of the photoconductive layer and the dielectric layerIpositive to offset the negative charge on the conductive plate 11. Theinduced negative charge is drained off" through the conducting screen orgrid While the positive charge is trapped at thephotoconductor-dielectric interface when the light source is turned offand the photoconductor reverts to its relatively insulating state.Switch 18 is then connected to ground, draining charge from thecapacitor but leaving the trapped charge at thephotoconductor-dielectric interface. The result is a positively chargedplate. With the screen or grid structure ystill connected to ground theplate is then exposed to the subject to be copied. Those areas of thephotoconductive layer which are exposed to light from the projectedlsubject again become conductive allowing the trapped charge in thoseareas to move laterally through the photoconductor to the grid structureand thence to ground effectively discharging those areas while leavingthe nonilluminated areas charged. Alternatively, the grid structure maybe connected to a negative potential source rather than to ground so asto move charge from the illuminated areas more quickly thus making theplate more sensitive. Thus, either plate may be used to trap charge atthe interface under uniform illumination put in the dark, grounded andexposed to the subject so as to form a latent electrostatic image oreither plate may have potential applied during exposure and placed inthe dark so that the ield emanating from the conductors in areas whichhad not been exposed may be developed.

The method outlined just above produces positive prints from theexposure of a positive original. In order to produce image reversal or anegative print from the exposure of a positive original, the steps ofthe method outlined just above are carried out e.g. uniformlyilluminate, apply potential, place in the dark ground and expose in thatorder and-then the same potential is reapplied during development sothat the eld set up in non-equipotential or unexposed areas isdeveloped. Thus, either method of image reversal copying may also beused with either plate.

Any one of the plates described heretofore may be exposed through itsback if it is provided with transparent base and dielectric layers. Theconductors must also be transparent if the whole area of the plate is tobe exposed to the image to be reproduced. However, even if theseconductors are opaque a halftone line pattern of exposure will getthrough to the photoconductive layer on exposure to the plate backing.In some instances this method of exposure to the subject is advantageousbecause it increases the plate speed. The advantage of using rearexposure with a plate such as that shown in FIGURES 3 and 8 will beclear when the -basic operation of a photoconductor is considered.

When the energy of a photon of light is absorbed by the photoconductorit produces a hole-electron pair making the photoconductive layerconductive. According to present theory this hole-electron pair isformed within the first few microns of the exposed surface of aphotoconductor such as selenium. Thus, with rear exposure of the plateas shown in FIGURE 8 using a projector 39 hole-electron pair formationoccurs close to the photoconductor-dielectric interface and in theregion of maximum field strength due to the closeness of the grid. Withfront exposure of this plate a weaker field would exist at the region ofhole-electron pair formation since this region would -be relatively farfrom the grid. In addition, path lengths for discharge would beincreased since the charge would have to move from the top surface ofthe photoconductor all the way through to its rear surface and theinterface with the dielectric of the grid.

In order to incorporate the basic concepts of this invention into anautomatic, rotary copying machine utilizing a cylindrical, hexagonal orother polygonally shaped xerographic plate, also referred to as a drum,proper switching circuits for plate segments are required. By way ofexample, FIGURE shows a very small segment or arc of a cylindrical drumsurface viewed from outside the cylinder. This iigure shows exemplaryconductor placement and switches needed for automating the FIG- URE 3plate embodiment. The drum includes a number of transverse conductors2,1, A22, and 23 parallel to the longitudinal drum axis. Perpendicularto, `and intersecting these transverse conductors are additionalconductors such as 24, 25, and 26. Each of these perpendicularconductors intersects only one transverse conductor and is interleavedbetween two adjacent perpendicular conductors on the adjacent transverseconductor. Each of the transverse conductors is connected to a contactat the drum end such as 27. It should be noted that these contacts maybe placed a either or both ends of the drium. During rotation of thedrum the contacts such as 27, etc., intermittently make connection withfixed external contacts such as 28, 29, etc. Each of these iixedexternal contacts is separate from the other external contacts and isconnected to various circuit elements such as the potential source 30, areverse potential source 85, etc. Thus, that portion of the plate in acharging zone is connected to the positive potential source, thatportion of the plate in an exposure zone is connected directly toground, that portion of the plate in a transfer zone is connected to apotential source of a polarity opposite to that of the source 30, etc.In adapting the FIGURE l plate embodiment for use in an automatic rotarycopying machine every other conductor in a plate zone would be connectedto a contact at one end of the drum while the conductors between thesealternate conductors in a plate area would be connected to a contact atthe opposite end of the drum. The FIGURE 3 e'mbodiment could also beadapted for use in a continuous automatic copying machine by making thephotoconductive grid and dielectric layers in the form of an endlessbelt and using a number of separated base plates like plate 11 of FIGURE3. Then different potentials may be applied to these plates each havinga different effect on the belt portion above it.

FIGURE 6 shows an end view of a hexagonal plate according to thisinvention utilized in a rotary copying machine. The hexagonal plate 40is made up of six photoconductive faces 41, 42, 43, 44, 45, and 46 eachincluding conductors as shown in the FIGURE l or FIGURE 3 embodiments.These photoconductive faces are over a dielectric layer 47 mounted onthe conductive drum backing 48. At the end of the drum an insulatingdisc-ring 49 supports slip rings 41a, 42a, 43a, 44a, 45a, and 46a whichare connected to the conductorsin the corresponding plate faces.

This figure also indicates how the drum would be moved through thereproduction steps. When using this hexagonal plate configuration theplate is operated in a stop and go manner being indexed around to eachsuccessive processing station as explained below. Each of theseprocessing operations is controlled by the indexing of the drum. Theindexing control may also include a counter so as to allow multipleprojections of the same original. With lthe plate stopped face 46 of theplate is charged from a potential source such as a battery through slipring 46a and an external fixed contact while face 41 is being exposed toan original to be reproduced by projector 50. Alternatively the exposuredevice may comprise a cathode ray tube or other image source. At thesame time face 42 having previously been charged and exposed is in theprocess of being developed by cascade developer 51 while face 44 is inthe transfer zone. The developed image on face 44 is in the process ofbeing transferred to copy sheet 52 backed up by `a grounded conductiveweb holder 65. It is later fixed by heating element 53. Since the copysheet must be backed up by a grounded conductor during image transfer asexplained elsewhere in this specification in connection with FIG- URE 9and since the copy sheet must be moved away from the plate when it isindexed around to prevent image destruction due to rubbing between thehexagonal plate and the copy surface a grounded conductive web holder 65is provided.- Both the copy web and the web holder are wider than thelength of the plate and the holder is provided with short overhanginglips `so it can pull the web away from the plate after the imagetransfer is cornpleted. These lips are short enough so that they dontcontact the plate when the web holder pushes the web against the plateprior to powder image transfer. The web holder 65 is reciprocated by apiston and cylinder arrangement 66 which is operated by the plateindexing control. A sectional view along line 6A of FIG. 6 through face43 of hexagonal plate 40, more clearly illustrating the relationshipbetween web holder 65, copy sheet 52, and piston-cylinder arrangement 66is shown in FIG. 6A. In this case transfer is achieved by theapplication of a potential of a polarity opposite to that used incharging the plate. This potential may be applied through an externalfixed contact to the internal plate conductors touching slip ring 44a,this method of transfer being more fully explained in connection withFIGURE 9 below. Face 45 is in the process of being cleaned of anyresidual developing material by spring mounted brush 54 so that it maybegin a new cycle. In addition to being hexagonal, the drum shown inFIGURE 6 could be of almost any other 'polygonal shape or cylindrical inwhich case it might be continually rotated through the processing steps.In place of the contacts 27-29 shown in connection with FIGURE 5, andthe broken slip ring 41a through 46a shown in connection with FIGURE 6,other various modifications for making sliding intermittent contactbetween the conductors in selected plate areas and external sources maybe utilized. For example, concentric slip rings, commutator segmentsseparated by insulators and other mechanisms familiar to those skilledin the art of rotating electrical machinery might be utilized.

FIGURE 7 shows a modified version of the plate disclosed in FIGURE 3.This modified plate has a photoconductive layer 31 and a grid structure32, 33, 34, etc., both of which are the same as the photoconductivelayer and grid structure of the FIGURE 3 embodiment. Underlying these isa dielectric layer 35 which is also similar to the FIGURE 3 embodimentexcept for the fact that the dielectric is filled withelectroluminescent phosphor of the type which may be excited toluminescence by the application of A.C. potential or pulsating D.C. Anyone of the standard luminescent phosphors may be used such as Blue AQ62-2861 manufactured by E. I. du Pont de Nemours Co. Below thisdielectric phosphor layer is a conductive base 36 similar to theconductive base of the FIG- URE 3 embodiment. By applying `an A.C.potential source such as 3'7 across the dielectric-phosphor layerthrough the grid structure and conductive backing by closing a switchsuch as 38 during the exposure step a bootstrap effect is achieved. Assoon as the A.C. potential is applied the phosphor directly below theconductive grid structure will begin to glow, however since theconductors in this embodiment are relatively opaque most of the lightproduced directly below the conductors will not reach thephotoconductive layer. Since all of the plate layers are quite thinincluding the layer containing electroluminescent phosphor, luminescingof the phosphor below these conductors has relatively little effect onthe photoconductive layer between the conductors. But when incidentlight strikes the area of the photoconductor between adjacent conductorsthis area will become conductive allowing A C. potential from the source37 to be applied across the phosphor dielectric through the grid andlaterally across the photoconductor between the adjacent conductors.This causes the phosphor between adjacent conductors to glow,reinforcing or further exposing the photoconductive layer above thesephosphors. Thus, a bootstrap effect is achieved and even when the plateis exposed to relatively low light level images it will become quiteconductive due to the glowing'of the phosphor below the photoconductivelayer after the initial exposure to incident light. Since the conductorsare opaque only areas between conductors are bootstrap exposed.

The novel transfer process utilized in connection with the new plate ofthis invention is shown most clearly in FIGURE 9. For purposes ofillustration this transfer method is shown in connection with the FIGURE3 plate embodiment; however, it can be used equally well with the FIGUREl embodiment plate as will be explained below. This plate which haspreviously been positively charged by applying the positive side of apotential source to conductors 14, 15, 16, 17, etc., exposed anddeveloped with negative electroscopic ydeveloping particles 56 is shownin this view with the potential source reversed. This is accomplished bychanging switch 18 of the apparatus of FIGURE 3 to a position so that itconnects potential source 55 across the plate thereby applying negativepotential to the grid structure 14-17 etc. This negative potentialserves to repel the negatively charged developing particles 56 away fromthe plate and to the copying surface 57, which is backed up by agrounded conductive plate 60 to prevent charge buildup. Actually thiscopying surface must be very close to or touching the plate at the timeof transfer and the wide gap between the plate and the copying surfaceis shown here only for purposes of illustration as are the greatlyenlarged toner particles. When this transfer method is used with theFIGURE l embodiment the switch 9 is moved so as to connect conductors 4,6, etc., to potential source 58. In this way a potential opposite inpolarity to that used to charge the plate is applied to transfer thetoner to the copy surface.

The voltage used in charging any one of the plates of this invention isnot critical, but as a general rule, increases in this voltage willresult in a more dense image. Thus, in order to secure more dense imagesthe voltage applied to the plate may be `increased up to the point wherebreakdown occurs in the dielectric layer. In testing these plates it wasfound that voltages of from 300- 600 volts produced satisfactory imageswith the images becoming denser as the voltages were increased. In viewof the relatively low current drain needed to operate the plates of theinvention ordinary B or photo-flash batteries may be used as a powersource. In testing these plates both Eveready 300 volt #493 and Burgess300 volt model number U-200 batteries measuring 2% x 29/32 x 3%" wereeffectively used, with two connected in series to get 600 volts.

One of the most gratifying results yof this invention was that largesolid dark image areas could be very uniformly developed. Conventionalxerographic plates have suffered somewhat in this respect because thelarge solid black image areas are represented on a plate by largeretained charge patterns which set up on electric field. As is wellknown to those familiar with development in xerography large electricfield areas are more effective at their peripheries or fringe areas thanat their centers and in xerographic plates the weak central field linesgo in towards the conductive plate backing. This results inelectroscopic developing particles being more strongly attracted tothese peripheral areas leaving the centersy of these large areasrelatively washed out or undeveloped. By using the screen or gridpattern as one of the conductive members or plates in the built-incapacitor of this novel xerographic plate these large area fields arebroken up into many small areas. This occurs when the field is caused bycharge trapped at the dielectric-photoconductor interference because thefield can only exist in the `areas between the conductors since anelectric field will decay when crossing a conductor. When the plate isoperated so that the field emanates fro-m the conductor itself-the fieldis broken up because a small separate field emanates from each smallconductor which is `separated from its adjacent conductors by a smalllayer-of dielectric material. Thus, regardless of how the plate isoperated or which plate embodiment isr used any large charged area willbe a composite of small discrete electric fields thereby eliminating thesolid area peripheral field effect.

The examples below are of the FIGURE 3 embodiment.

EXAMPLE 1 A two mil thick Mylar sheet was coated with an opaque layer ofcopper by evaporation. The copper coated Mylar was then flow coated withKodak Photo- Resist hereinafter referred to as KPR. This was air driedand exposed to a parallel line pattern of approximately l0() lines perinch lwhich was 50% transparent. The KPR was then developed using astandard KPR developer which hardened the exposed portion of the KPR.The unexposed portion of the KPR was then washed away and the copperwhich was then exposed was etched away, leaving a conductive copper linepattern on the Mylar. The completed base was then coated with a l2micron layer of amorphous selenium by evaporation. The plate was thentaped to an aluminum plate base fo use in the tests which produced goodquality prints.

EXAMPLE 2 An aluminum plate was flow coated with a contact cement andallowed to dry. A 1A mil Mylar sheet was bonded to the plate withpressure. The Mylar layer was then coated with copper and KPR in themanner explained in connection with Example 1 above. The KPR was thenexposed to a line per inc-h pattern and developed as explained inExample l above. Etching of the copper 4was also carried out in themanner explained in -connection with Example l above. 4This completedbase was then coated with a layer of amorphous selenium having athickness of approximately 5 microns. Voltages of 300 and 600 volts D.C.were used to charge the plate. Both voltages produced acceptable images,however the higher voltage produced prints of higher maximum density.These voltages were secured from one Burgess U-200 300 volt battery andtwo such batteries connected in series. A second mode of operation wasalso used with this particular plate. This involved applying the biasvoltage only during subject exposure as explained above in connectionwith positive to negative copying with the FIGURE 3 embodiment. Thisresulted in good quality negative images of the subject.

Developed images were transferred from the plate to a paper copy sheetusing both conventional corona discharge transfer equipment and byreversing the bias applied to the plate in the charging step. The biasreversal transfer was carried out as follows: A sheet of paper was laidfiat on the developed image of the plate. A grounded aluminum plate waslaid on top of the paper thus forming a sandwich. A potential of apolarity opposite to that used in the charging step was applied to theplate resulting in a successful transfer to the copy sheet.

EXAMPLE 3 A plate was fabricated in accordance with the steps of Example2 except the transparent materials were used for the conductive base andthe dielectric layers. In this case NESA glass was used as a base. Thismaterial which is available from the Pittsburgh Plate Glass Company ofPittsburgh, Pennsylvania, is believed to be a glass base covered with athin conducting layer of tin oxide. This conductive base material wasthen fiow coated with an epoxy resin solution which was then allowed toharden. The conductive line pattern and the selenium coating wereapplied in the same manner as explained in connection with Example 2. Aselected front exposure of this plate produced underexposed prints,however, the same exposure when applied to the rear of the plate throughthe transparent backing and dielectric layers resulted in an overexposedprint. This confirms the theory proposed in connection with rearexposure of the FIGURE 3 embodiment.

It should be recognized that many alternate materials and configurationsmight be utilized in constructing a xerographic plate or drum inaccordance with the concept of this invention. For example, conductivebase materials might include aluminum, brass, copper, NESA glass, etc.,while dielectric materials might include Mylar, glass, Bakelite, certainepoxy resins, etc. Alternate photoconductive insulating materials mayalso be used such as amoprhous selenium, zinc oxide in an insulatingbinder, or cadmium sulfide. In addition to the coppercoatingphotoresist-etching technique for applying the line or gridpattern, this pattern can be engraved on a glass base using a mechanicalengraver after which the grooves produced are filled with a conductivecomposition such as finely divided metal or graphite powder in asuitable binder.

The pattern of the conductors and their concentraiton may be varied fromabout 50 lines per inch to about 300 or 400 lines per inch depending onthe desired resolution. In fact, even plates outside these limits couldbe used if this was found to be desirable. In view of the many possiblemodifieations of this invention, some of which have been illustratedabove, it is intended that all matter contained in the above descriptionbe limited only as defined in the appended claims.

The term xerographic plate as used in this specification and theappended claims should be understood to refer to a device capable oftaking and holding an electrostatic charge, and of dissipating portionsof said charge in accordance with an image of activating radiation towhich it is later exposed without regard to its shape. In other words,the term xerographic plate includes flexible or rigid members, fiatplates, cylindrical drums, spheres, or other surfaces t of whateverconfiguration.

The term screen as used in this specification and the appended claimsshould be read in its broadest sense. For example, the screen mightinclude `such diverse configurations as a perforated plate, Ia meshedfabric similar to the common window screen, a spiral with closely spacedadjacent convolutions, a number of narrow closely spaced parallelmembers, etc.

The term image reversal as used in this disclosure shall be understoodto refer to that term as it is generally used in the photographic arts.The effect of image reversal is a negative copy where the original ispositive and vice versa.

What is claimed is:

1. A xerographic sensitizing process comprising applying a potentialacross two dielectrically separated conductive members one of which is afine conductive screen, said screen being contiguous with aphotoconductive layer so as to set up electric fields through saidphotoconductive layer, said two dielectrically separated conductivemembers lying on one side of a surface of said photoconductive layer,with at least a portion of said dielectric being in contact with saidphotoconductive layer, while uniformly subjecting said photoconductivelayer to activating radiation, whereby competing electric fields areestablished in the radiation struck areas, removing said source ofactivating radiation and removing said potential source from saidconductive members whereby charge is trapped at the photoconductivedielectric interface.

2. A method according to claim 1 further including grounding saidconductive members and exposing said sensitized plate to an image to becopied with a source of activating radiation and then developing saidplate with finely divided electroscopic material.

3. A method according to claim l further including grounding saidconductive members and exposing said sensitized plate to an image to becopied with a source of activating radiation and then developing saidplate with finely divided electroscopic material.

4. A xerographic method comprising applying a potential source acrosstwo dielectrically separated conductive members, one of said conductivemembers comprising a fine conductive screen continguous with aphotoconductive layer so as to set up electric fields through saidphotoconductive layer, said two dielectrically separated ccnductivemembers lying on one side of a surface of said photoconductive layer,with at least a portion of said dielectric being in contact with saidphotoconductive layer, while exposing the photoconductive layer to animage to be copied with -a source of activating radiation, wherebycompeting electric fields are established in the radiation struck areas,removing the source of activating radiation and developing said platewith finely divided electroscopic material.

5. A method according to claim 4 further including grounding saidconductive members prior to development whereby a reversed image will bedeveloped.

6. A xerographic apparatus comprising two conductive members, adielectric material separating said two conductive members, at least oneof said conductive members being a fine screen, a photoconductive layercontiguous to said conductive screen, said conductive members and saiddielectric material all lying on one side of a surface of saidphotoconductive layer with at least a portion of said dielectric layerbeing in contact with said photoconductive layer, 'and means to apply anelectric potential to said screen capable of creating a significantpotential difference within said screen, whereby electric fields areestablished through said photoconductive layer.

7. A xerographic apparatus according to claim 6 in which said fineconductive screen is made up of a group of closely spaced slenderconductors.

8. A xerographic apparatus according to claim 7 including a second groupof closely spaced slender conducaassoa tors intersecting said firstclosely spaced slender conductors of the conductive member so as to forman electrically continuous grid-like structure.

9. A Xerographic apparatus according to claim 6 including means toswitch the conductive members from the potential source to groundwhereby they may be discharged.

10. A Xerographic apparatus according to claim 6 including means toreverse the polarity of the applied charging potential and a groundedconductive plate near the phtoconductive surface layer whereby areversal of potential Ipolarity may be used to transfer electr-oscopicparticles from a developed Xerographic plate to a copy surface betweenthe grounded conductive plate and the Xerographic plate.

11. A xerographic apparatus according to claim 6 in which the conductivescreen is embedded in the photoconductive layer.

12. A xerographic apparatus according to claim 6 in which the conductivescreen is embedded 'in the dielectric layer and separated from the otherconductive member by at least -a portion of said dielectric layer.

13. Apparatus according to claim 7 in which both conductive members areine screens, and the conductors of one screen are interleaved betweenthe conductors of the other screen, said screens being separated fromeach other by at least a portion of said dielectric material.

14. Apparatus according to claim 13 in which said screens are separatedfrom said photoconductive layer by at least a portion of saiddielectric.

15. A Xerographic apparatus according to claim 6 in which both of theconductive members are dielectrically separate-d screens and in whichthe photoconductive layer is in contact with the dielectric material andelectrically separated from the conductive screens and in which thepotential applying means comprises means to apply potential of oppositepolarity to each of the screens.

16. A xerographic apparatus according to claim 6 including a llin-g ofelectroluminescent material in said dielectric layer and means to applyan alternating current potential across the two conductive members onopposite sides of said filled dielectric, the lscreen shaped conductivemember being at the interface of the photoconductive 4layer and thedielectric layer.

References Cited by the Examiner UNITED STATES PATENTS 2,277,0133/119442 Carlson 96-1 2,808,328 10/1957 Jacob 96-1 2,836,766 5/1958Halsted 96-1 2,892,709 6/1959 Mayer 96-1 2,909,971 l10/1959 Barber95-1.7 2,917,385 12/1959 Byrne 96-1 2,946,682 7/1960 Lauriello 96-12,947,625 8/ 1960 Bertelsen 96-1 2,968,553 1/'1961 Gundlach 96-12,984,163 5/1961 Giaimo 95-1.7 3,000,735 9/1961 Gunning et al. 96-13,003,869 10/196'1 Schaffert 96-1 3,005,707 10/1961 Kallrnan et al 96-13,062,110 11/1962 Shepardson et al 95-1.7 3,137,762 6/1964 Baumgartneret al. 96-1 X NORMAN G. TORCHIN, Primary Examiner.

A. L. LIBERMAN, D. PRICE, Assistant Examiners.

1. A ZEROGRAPHIC SENSITIZING PROCESS COMPRISING APPLYING A POETENTIALACROSS TWO DIELECTRICALLY SEPARATED CONDUCTIVE MEMBERS ONE OF WHICH IS AFINE CONDUCTIVE SCREEN, SAID SCREEN BEING CONTIGUOUS WITH APHOTOCONDUCTIVE LAYER SO AS TO SET UP ELECTRIC FIELDS THROUGH SAIDPHOTOCONDUCTIVE LAYER, SAID TWO DIELECTRICALLY SEPARATED CONDUCTIVEMEMBERS LYING ON ONE SIDE OF A SURFACE OF SAID PHOTOCONDUCTIVE LAYER,WITH SAID PHOTOCONDUCTIVE LAYER, WHILE UNIFORMLY CONTACT WITH SAIDPHOTOCONDUCTIVE LAYER TO ACTIVATING RADIASUBJECTING SAID PHOTOCONDUCTIVELAYER TO ACTIVATING RADIATION, WHEREIN COMPETING ELECTRIC FIELDS AREESTABLISHED IN THE RADIATION STRUCK AREAS, REMOVING SAID SOURCE OFACTIVATING RADIATION AND REMOVING SAID POTENTIAL SOURCE FROM SAIDCONDUCTIVE MEMBERS WHEREBY CHARGE IS TRAPPED AT THE PHOTOCONDUCTIVEDIELECTRIC INTERFACE.
 6. A XEROGRAPHIC APPARATUS COMPRISING TWOCONDUCTIVE MEMBERS, A DIELECTRIC MATERIAL SEPARATING SAID TWO CONDUCTIVEMEMBERS, AT LEAST ONE OF SAID CONDUCTIVE MEMBERS BEING IN A FINE SCREENA PHOTOCONDUCTIVE LAYER CONTIGUOUS TO SAID CONDUCTIVE SCREEN, SAIDCONDUCTIVE MEMBERS AND SAID DIELECTRIC MATERIAL ALL LYING ON ONE SIDE OFA SURFACE OF SAID PHOTOCONDUCTIVE LAYER WITH AT LEAST A PORTION OF SAIDDIELECTRIC LAYER BEING IN CONTACT WITH SAID PHOTOCONDUCTIVE LAYER, ANDMEANS TO APPLY AN ELECTRIC POTENTIAL TO SAID SCREEN CAPABLE OF CREATINGA SIGNIFICANT POTENTIAL DIFFERENCE WITHIN SAID SCREEN, WHEREBY ELECTRICFIELDS ARE ESTABLISHED THROUGH SAID PHOTOCONDUCTIVE LAYER.